channel: add deprecation message (#78)

The Channel API feature has been deprecated and is going to be removed.
See the Channel API Turndown document for details and timetable.

https://cloud.google.com/appengine/docs/deprecations/channel
diff --git a/channel/channel.go b/channel/channel.go
index dfe0a3f..96945f6 100644
--- a/channel/channel.go
+++ b/channel/channel.go
@@ -17,6 +17,10 @@
 Send sends a message to the client over the channel identified by clientID.
 
 	channel.Send(c, "player1", "Game over!")
+
+Deprecated: The Channel API feature has been deprecated and is going to be removed. See the Channel API Turndown document for details and timetable.
+
+https://cloud.google.com/appengine/docs/deprecations/channel
 */
 package channel // import "google.golang.org/appengine/channel"